I was in a hurry and short on cash so I "temporarily" cut a 4 or 5mm thick piece of plexiglass ($8), bent it with hot air and placed it in my car thinking that a few weeks later when it will be worn out I would replace it with a proper window.

It's been closing in two years and it is still crystal clear (which is why I kinda forgot about it), though with microscratches if you look really close. Still transparent as ever.

I'm bringing this up because yesterday two guys on a bike attempted to steal my briefcase. Stopped right next to me on a red light and one of them tried to smash through the glass with a hammer. The tool bounced right off the window and hit the driver of the bike on the helmet. They almost fell and fled ungracefully.

For a very long time I thought that plexiglass is not fit for automotive purposes but now I am reconsidering.

Apart from the windshield (which would degrade faster due to direct impact of microparticles), what do you think are the reasons that plexiglass is not considered for car windows?

I wonder if it's breakable when the car is underwater... if it's unbreakable, then I guess it'll backfire and significantly reduce the safety of the driver when they are trapped inside a submerging car.
– Andrew T.Jun 3 '16 at 6:21

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It should be commented that plexi and other plastics are used in car windows, just not road car windows. Many race cars have plexi windows for weight saving purposes as well as safety purposes (less shattered pieces of glass all over the track). However as a rule of thumb, what passes on a racetrack doesn't generally pass on the road.
– Aaron LaversJun 3 '16 at 7:45

Rear quarter windows made of plastic do exist. Those are normally fitted for life, not moving, not opening.
– GáborJun 5 '16 at 19:52

@AndrewT. You could always wait until the car is fully submerged and then open your car door which was made widely known thanks to Mythbusters.
– phkJan 8 '17 at 11:25

9 Answers
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Plexiglass is flammable. While it doesn't release toxic gases or excessive amounts of smoke, it is still rated B2 (normally flammable) and thus forbidden as interior material in motor vehicles, including windows.

Collision behaviour mentioned by Thomas is also a crucial property, especially for bigger pieces like windows. If that guy on a bike had a bigger hammer, he could have popped your window inside the vehicle, where its shards would have released their kinetic energy by hurting the driver and the passengers. The same thing could have happened on a side collision with another car.

didn't think about it like that. Interesting input. The link you posted was particularly helpful.
– PaulBMay 31 '16 at 18:35

18

Another point to consider. Emergency responders are trained to extract motor incident victims through the quickest means possible. When the door is jammed, that often means the breaking the window. I.E The windows were DESIGNED to break, and in the case of the windscreen, break in a very specific way.
– AronJun 1 '16 at 5:45

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@Aron Plexiglass is not an issue for rescue teams, it can be cut rather easily with appropriate tools. It will be an issue for the driver who needs to escape his car which is burning or drowning, though - the driver is likely to have very few tools at his disposal, a car window breaker at best.
– Dmitry GrigoryevJun 1 '16 at 7:33

9

Also, Plexiglas is very common for light aircraft. The reason is weight. However, this comes at a downside. It will scratch much more easily than tempered auto glass. It is not uncommon to see an older aircraft window that is full of small scratches, making poorer visibility.
– JoeJun 1 '16 at 11:34

Because of how it reacts in a collision. Auto glass is designed to shatter into small bits that are fairly dull, as opposed to large, sharp daggers. In the US, your car must be outfitted with glazing material that meets certain specs, and material type is not specified. If anyone could get plexiglass to satisfy those specs, I am sure it would be lower cost and popular. So I assume plexiglass cant meet the specs.

Also, might have to do with the gasses released if it is melted.

Aside from that, over time plastic fogs from UV. Most headlights these days are plastic. One of the first things some enthusiasts do is replace them with glass. When you look at cars barely 10 years old with fogged and pitted headlights, that is exactly what plexiglass would look like it it was used on the windows.

Would it not be possible to create a combo windshield, thin sheet of plexiglass at the top of glass or the opposite. Would that not get rid of the shattering issue and retain elasticity?
– AvamanderMay 31 '16 at 16:04

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Yes but the headlights are plastic not acrylic. Plexiglass is acrylic and does not become yellowish as time goes by... At least not as fast as plastic. As i said i had it for about two years and there is no hint of loss in transparency. The only issue is with scratches. I think the issue is as you mentioned with collision reaction, where the glass HAS to break instead of having people smash their heads on it...
– PaulBMay 31 '16 at 18:32

@AnthonyX I'm not convinced that planes have exactly the same safety considerations and weight is a much bigger factor, there.
– David RicherbyJun 1 '16 at 10:52

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@jameslarge Um. You have it completely 100% backwards. Assuming you're in a country with a population of more than a few million, multiple people die in car crashes in your country every single day of the year and everybody says, "Well Duh! It was a car crash. What did you expect?" In contrast, every single plane crash in a developed country causes a government investigation that tries to find out "how could this have happened?" and "what could we have done differently?" It's plane crashes that get people worked up, not car crashes.
– David RicherbyJun 3 '16 at 14:07

Used to be, when not all cars in hot climes had air conditioners, you could tell those drivers who did not because they'd roll down the driver's side window and sometimes put their elbow on the door. The left arm would tan while the right arm, under the UV blocking glass would not.

In the third world, they're taking advantage of most plastics UV transparent to sterilize water. They just fill up an old hand-sized water bottle and toss it it up on a corrugated tin roof for a week or so and when they take it down, all the microbes have been fried. Somebody should have gotten the nobel prize for that.

I've gotten sunburns in cars even with the windows up.
– JABJun 2 '16 at 17:33

2

Another thing to mention is that plastics can be easily made to filter out UV if it's a serious issue.
– I have no idea what I'm doingJun 3 '16 at 11:17

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@JAB That's because you were driving a convertible with the roof down. :-P More seriously, your skin can still turn red from overheating when exposed to the sun, even if the UV is filtered out. When that happens, it initially looks a lot like the early stages of sunburn but it's not sunburn.
– David RicherbyJun 3 '16 at 14:23

Plastics are awesome. One of their characteristics is that they easily degrade in UV / sunlight. In my experience that is the weakest link in its usage. With that said, there are a whole lot of plastics used on modern cars.

Generally headlamp lenses are made of polycarbonate. The polycarbonate material has a thin coating sprayed on top of the lens to help provide extra UV protection.

As for why plexiglass is not used in cars? My guess is you really need to follow the dollar bill $$$$$$$$. Glass forming dies aren't nearly as expensive as molds necessary for things like headlamp lenses. The molds (Dies) for these products is crazy expensive. You can't imagine.

I did a whole lot of engineering for a US based Original Equipment Manufacturer (OEM) for both exterior glass and exterior lighting products. I will say the molds for polycarbonate headlamp lenses were some of the most highly engineered (and super expensive) tooling we had to deal with. Because of the size of the molds you really can only have one person working on a mold at a time. Its not possible to put ten people on the job and have it go ten times faster. That steel is super expensive grade, super hard. If you do something wrong or try to go too fast you can crack a mold.

Some of those tools can take up to year to produce from start of design till first parts come out of the press. From my experience, headlamp design and tooling is on the critical path for a vehicle's development cycle.

No way you'd want that hassle or tooling investment cost for door glass or windshields or rear window. Float glass is still expensive if you had to build up a glass factory from scratch, but once the factory is built glass is very reasonably priced. Heck, the raw material for glass is essentially sand. Float glass has been around for a long time. True you can only shape glass as a ruled surface but designers have worked with that for a long time.

And glass doesn't degrade in UV light. Not one bit.

For the question "Why did the manufacturer do that?" the answer is often.. because its cheaper that way. (Follow the money!)

I think in answering this question you need to answer the initial question which is fundamentally, is plexiglass used for glazing in road cars. The answer to this question is that it is a very definite yes, it is used.

One example of a production vehicle that uses plastic windows is the Porsche 911 GT3 (991 GT3 R). Taken from wiki information about the car available here

All windows – and for the first time ever, the windscreen – are made from polycarbonate to cut weight

Local legislation may prevent the use of plexiglass for road car windows, which is reflected in the same article which states

The American version ... has a standard rear window (not plexiglas) ... to comply with rules of SCCA

Ths 911 GT3 was the first example I thought of but some further research reveals that more humble road cars, such as the Smart Fortwo roadster uses plastic windows.

Polycarbonate and plexiglass are not identical. In fact Plexiglas(s) is a trademark name for PMMA. Polycarbonat is the tougher of the two, and thus better suited for impact protection and less prone to crack.
– Yves KlettJun 3 '16 at 16:16

Can you elaborate on this? It's a good start to an answer, but more detail would improve it - for example, why is scratching bad? What causes it to scratch? etc...
– Nick C♦Jun 1 '16 at 14:40

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I voted to keep this for now to give the poster a chance to elaborate.
– cdunnJun 1 '16 at 17:22

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Surely glass is fairly fragile and can not only be scratched but cracked, shattered and holed and that's suitable for road use.
– Steve MatthewsJun 3 '16 at 14:50

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@SteveMatthews, glass has a Mohs hardness of around 5.5 -- harder than most of the road debris you'll encounter (it can still be scratched by quartz sand, though). Most plastics don't even register on the Mohs hardness scale.
– MarkJun 3 '16 at 22:22

@SteveMatthews Glass is fragile in that when it fails, it fails thoroughly, but this requires an impact of significant force, especially with tempered glass. Unless that happens though, it's virtually immune to regular wear. Plastics, however, can be scratched by common dirt, and etched with household chemicals - as people who clean plastic-lensed glasses with their shirt or windex learn.
– millimooseJun 5 '16 at 20:54

Apart from collision/fire behavior, automotive glazing has to resist damage from road debris - stones and such. It also has to resist abrasion from dusty/dirty windshield wipers. That requires a hard material which plexiglass is not. As well, you need an undistorted view of the road ahead, which requires that the windshield be of uniform thickness and free of ripples. That can be hard to do with plastic.

In Israel people who live in dangerous areas can have their car windows replaced for free. They replace the front windshield with bullet proof glass. The side windows are replaced with some type of plastic to protect from rock throwing. The side windows do scratch and turn a yellowish color after a few years and then they can be replaced.

One insight I got in a similar question about house windows on a different SE site:

The assumption that you want unbreakable windows in a car might be wrong. If you crash your car in a way that jams your doors - e.g. someone rams you against a wall from the side - the side and rear windows provide a quick method of escape if you have an emergency hammer designed to break through the tempered glass used in side windows. (Which is designed to shatter into small, blunt shards.) The windshield is made out of laminated glass which won't break anyway, precisely so road debris doesn't destroy it.

Thank you for your interest in this question.
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